There are two possibilities:
A) Electrical- the teleconverter may lack the capability of transmitting the focus commands and receive the information from the lens for focus confirmation, making it necessary to take AF out of the loop by selecting MF on the lens.
B) The max aperture of your lens is 5.6, and the 7D has a tough time discerning an edge with which to determine focus at apertures smaller than 5.6, and when you put a teleconverter on that or any other lens, the max aperture gets smaller. If I put my Canon 2XII converter on my 70-200 f/2.8L IS, the max aperture is 5.6 as it is a 2x converter. When you employ a TC, there are two downsides: Speed of the glass and loss of Image Quality, no matter whose TC you use. My 7d has a problem autofocusing at apertures smaller than 55.6, although the spec says it should be able to AF at f/8. It is a light issue and the AF sensor being able to find an edge or high contrast area which to focus on.
Hope it helps.